Measurement problem in simple experiment

[PeterDonis]

Of course you can always reverse this in QT (unitary transformation are reversible)

But that does not mean that what happens when a measurement is made and a permanent record is formed (even if it's not readable by humans, for example if it's stored in a large number of untrackable degrees of freedom that do not include any "pointer" or other indication that humans can read) can be reversed. Our actual experimental observation is that it can't; nobody has ever reversed decoherence, nor is there any prospect of anyone doing so. Certain interpretations of QM might claim that decoherence can in principle be reversed, but that is not the same as having actual evidence that it can.

 

[PeterDonis]

once the slit has been marked by an interaction, the interference pattern at the screen will not occur.

"Marked by an interaction" is not the same as "interacted with qubits". If you want to claim that interaction with a single qubit at one of the slits can suppress the interference pattern, please give a reference to a paper describing an experiment that has shown this.

 

[gentzen]

This doesn't seem consistent with e.g. "Theory of Decoherence" (section 1) explanation here.

This is a philosophy website, not a physics textbook or peer-reviewed paper. See further comments on that below.

decoherence occurs if the electron interacts with something at the slits that suppresses interference

By that they mean (though they don't say so--see my comment below about philosophy websites and physics) [... large number of degrees of freedom ...] by which they mean "results in no interference pattern being observed".

You are mistaken in believing that they mean [... large number of degrees of freedom ...], but you are right that they mean "results in no interference pattern being observed":

Such a phenomenon of suppression of interference is what is called decoherence.

This was somewhat surprising for me, because I expected "decoherence" to be the same as "environmental decoherence". For example, decoherence expert Maximilian Schlosshauer in 2011 gave the following definition in a Glossary:

decoherence A quantum-mechanical process whereby interactions of a quantum system with its environment lead to uncontrollable and practically irreversible entanglement between the two partners. Decoherence explains why it is so difficult in practice to prepare certain quantum states and to observe interference effects -- especially in the case of mesoscopic and macroscopic systems, for which decoherence is extremely fast and virtually inescapable. Decoherence is an application of the standard quantum formalism to open quantum systems; as such, it is neither an interpretation nor a new theory. Yet it is often invoked in foundational discussions, for example, when addressing aspects of the measurement problem. It’s also a cornerstone of Everett-style interpretations. Decoherence is a lively subject of experimental investigation and a feared enemy of quantum computers.

However, he also wrote in the same Glossary:

The Stanford Encyclopedia of Philosophy, online at http://plato.stanford.edu, is also an authoritative source of information. It has comprehensive entries -- some written by our interviewees -- on staples such as EPR, the Bell and Kochen–Specker theorems, the measurement problem, entanglement, quantum information, decoherence, quantum logic, and the common interpretations (Copenhagen, Everett, collapse theories, Bohmian mechanics, and modal and relational interpretations).

Interestingly, the first version of that SEP entry from 2003 did include "environment" as part of decoherence:

It is this phenomenon of suppression of interference through suitable interaction with the environment that we refer to by ‘suppression of interference’, and that is studied in the theory of decoherence. For completeness, we mention the overlapping but distinct concept of decoherent (or consistent) histories.

In 2012 Bacciagaluppi included sections on decoherent histories in the entry and weakened his statement to "It is this phenomenon of suppression of interference through suitable interaction with the environment that we call ‘dynamical’ or ‘environmental’ decoherence." Finally in 2020, he fully embraced that "suppression of interference" is what is meant by "decoherence" (if further qualifications are omitted).

My impression is that while the inclusion of "decoherent histories" might have been Bacciagaluppi's personal decision, there was also a real shift in the meaning of "decoherence" over time. What you can measure is the "suppression of interference" (within a well defined subsystem), so as control of decoherence became important for quantum computers and other quantum technologies, it made sense to separate the well defined "measurable" concept from the less well defined "explanatory" concept.

 

[PeterDonis]

My impression is that while the inclusion of "decoherent histories" might have been Bacciagaluppi's personal decision, there was also a real shift in the meaning of "decoherence" over time.

If there has been such a shift as you describe, it means that the term "decoherence" as it is now being used is not relevant to the measurement problem, since "decoherence" as it is now being used is no longer considered to be irreversible (since it is easy to reverse the operations in quantum computing that "suppress interference"). All of my statements about "decoherence" in this thread apply to the "environmental decoherence" concept which is what the term "decoherence" used to mean. If it doesn't mean that now, I'll change the term I'm using, not the arguments I'm making, which are still valid for "environmental decoherence" (or whatever we're supposed to call it now).

 

[gentzen]

but suppression of an interference pattern on the screen only requires e.g. qubits to mark the slot through

Can you describe a specific experimental setup in which that statement is true?

You need at least one qubit per electron to suppress an interference pattern in that way.
(a) The most straightforward experimental setups would be those that use quantum degrees of freedom of the electrons themselves for those qubits. Both electron spin and electron energy suggest themselves, but neither is easy/obvious to manipulate locally at a slit. (And since the electron wavelength is very small even at low energies, the "slits" must be very small and close to each other too, making local manipulations even more impossible.)
(b) Less straightforward experimental setups could try to use quantum degrees of freedom of random molecules put it the way of the electrons for those qubits. But this is problematic, because the interaction with such molecules risks to destroy any interference pattern, even in cases where no which-way information was ever present in the entanglement with those molecules.

once the slit has been marked by an interaction, the interference pattern at the screen will not occur.

"Marked by an interaction" is not the same as "interacted with qubits". If you want to claim that interaction with a single qubit at one of the slits can suppress the interference pattern, please give a reference to a paper describing an experiment that has shown this.

If "marked by an interaction" means "entangled with the state of some qubit", then the statement is not really false, but just misleadingly abstract. See my attempt above to describe some experimental setup in which that statement would be true.

 

[kith]

What you can measure is the "suppression of interference" (within a well defined subsystem), so as control of decoherence became important for quantum computers and other quantum technologies, it made sense to separate the well defined "measurable" concept from the less well defined "explanatory" concept.

The measurable concept sometimes goes by the name of "dephasing" (see p.9 of this for an attempted clarification of the nomenclature).

 

[gentzen]

If there has been such a shift as you describe, it means that the term "decoherence" as it is now being used is not relevant to the measurement problem, since "decoherence" as it is now being used is no longer considered to be irreversible (since it is easy to reverse the operations in quantum computing that "suppress interference").

I guess the difference between "irreversible" and "not reversed, maybe accidental, maybe intentional, or maybe because fundamentally impossible" is less important for (most) practical purposes than was once believed. What is important for decoherence in quantum computing is whether it is actually suppressed (by error correction methods), not whether is would have been possible (or easy) to suppress it.

And the "shift" might be less in the usage of the word decoherence, but in stopping to complain about or apologize for it. The passage in the document @kith linked above (2006, 2009) or the following passage at the end of chapter 8 in Mike and Ike (2000, 2010) are examples of such complaints/apologies:

An unfortunate confusion of terms has arisen with the word ‘decoherence’. Historically, ... The major point of these studies has been this emergence of classicality due to environmental interactions. However, by and large, the usage of decoherence in quantum computation and quantum information is to refer to any noise process in quantum processing. In this book, we prefer the more generic term ‘quantum noise’ and tend towards its usage, although occasionally decoherence finds a proper place in the context.

To conclude, please just continue to use the word decoherence as you did before, and rely on the context to disambiguate whether it means "environmental decoherence" or just "suppression of interference".My intention was never to complain about your usage of the word decoherence. Instead, I was bothered by the contrast between your "a philosophy website, not a physics textbook or peer-reviewed paper", Schlosshauer's "Stanford Encyclopedia of Philosophy ... is also an authoritative source of information. It has comprehensive entries -- some written by our interviewees ..." and the "unexpected" content and terminology of that SEP entry. (Of course, SEP cannot be an authoritative source of information for a physics forum. But even if it was, it could not substitute personal understanding and background knowledge. And an encyclopedia is not a good source for learning the basics either.) However, being the author of an SEP entry is more prestigious than being the author of a peer-reviewed paper. The first thing I did (before trying to understand the confusion of terminology) was to check whether Guido Bacciagaluppi, the author of that SEP entry, deserved that honor. After learning that he is the coauthor of Quantum Theory at the Crossroads: Reconsidering the 1927 Solvay Conference (arXiv link), I decided that he probably did, because deep knowledge about historical developments seems to be highly valued by SEP. (He is also one of the interviewees in Schlosshauer's book.) Let me also add that I know more about philosophy or quantum computing than I know about decoherence. So my guess that there has been that "shift" should be taken with a grain of salt.

 

[kith]

There might have been a shift in the usage of term "decoherence" but I think the more important thing is that there are two different cultures using a similar concept.

For the foundations community, the irreversibility is the most important aspect because they are interested in the quantum-to-classical transition. For the applied communities -like that of quantum information, molecular dynamics, quantum chemistry, etc.- the measurable consequences are the most important aspect. For them, coherence can be both lost and revived, depending on the experimental setup.

Personally, I dislike the restriction of "decoherence" to the irreversible case because it gives the illusion that we can easily distinguish between the irreversible and the reversible case while it might actually be a matter of experimental sophistication or practicality. I like to have the fuzzyness of the term "irreversible" explicit in order to connect with the issues which arise already in classical physics.

 

[PeterDonis]

What is important for decoherence in quantum computing

Quantum computing deals with qubits, not macroscopic objects. The measurement problem doesn't even come into play; all operations are unproblematically unitary and reversible. So quantum computing is irrelevant to this thread, which is about the measurement problem. Which, to me, means the usage of the term "decoherence" that you have given, as it is applied to quantum computing, is also irrelevant to this thread. Nobody has ever manipulated macroscopic objects the way quantum computing manipulates qubits.

Schlosshauer's "Stanford Encyclopedia of Philosophy ... is also an authoritative source of information.

No source of information is "authoritative". No source is always right. A philosophy encyclopedia might be a good source for discussions of philosophy, but one would not expect it to necessarily be a good source for discussions of physics. Physics is an experimental science; philosophical speculations play a role in physics, but physics is still a different discipline from philosophy. And this forum is for discussing physics.

being the author of an SEP entry is more prestigious than being the author of a peer-reviewed paper.

Physics is not a matter of prestige. It's a matter of finding models that make accurate predictions.